1. Field of the Invention
The present invention relates to an optical amplifier. Specifically, the present invention relates to an optical amplifier which uses the evanescent light for controlling an optical output level.
2. Description of Related Art
Information communication systems which convert data into an optical signal, use an optical fiber as a transmission path for sending and receiving data are in widespread use. A transmission path using an optical fiber has advantages in signal attenuation amount and a communication capacity comparing with a case where data is sent and received as electric signals.
In a transmission path using an optical fiber, to increase a communication capacity further, the optical wavelength division multiplexing (WDM) communication system is used. The optical wavelength division multiplexing communication system is a communication system for increasing the communication capacity by sending optical signals of different wavelengths simultaneously via a single optical fiber. Further, though the signal attenuation amount is small in the optical communication comparing with the case of sending and receiving data by an electric signal, for communicating through a large distance, an amplification of the optical signal is required at a specific interval. In the amplification of the optical signal, the optical output level of an optical amplifier is required to be controlled to be a constant level. For controlling the optical output level, the following equation is used.Palc[dBm]=Pch[dBm]×(10×log(Nch))  [EQUATION 1]
In the above equation, Pch [dBm] is a target value of the optical output level per 1 channel and is able to be determined to be desirable value by the designer. Nch is the channel number (multiplicity of wavelengths) of the signal light and means the number of wavelengths included in a multiplexed signal light. Palc [dBm] is a control target value of the optical output level, and to determine Palc [dBm], the detection of the channel number is required as represented in the above equation.
In Japanese Patent Application Publication JP 1996-335913A, an invention of an optical amplification monitoring device is disclosed, which detects the number of signal lights by using an inter-modulation light occurring in a case where signal lights of different wavelengths exist. In the invention disclosed in JP 1996-335913A, an exemplary embodiment is disclosed, in which the inter-modulation light is branched by a branching coupler and detected by a photodiode. Further, an exemplary embodiment is disclosed, in which only the inter-modulation light is extracted by reflecting and removing the signal light by an optical fiber grating, and the extracted inter-modulation light is detected by a photodiode.